1. Field of the Invention
This invention relates to an optical system of an information recording/reproducing apparatus for a magneto-optic disk, in which part of an area on a recording body is heated by a laser beam in order to weaken an anti-magnetic force, and in that state, an outside magnetic field is applied to invert the magnetization of that particular area so that information can be magnetically recorded, information being photoelectrically read and rewritten using the magnetic Kerr effect.
2. Description of the Prior Art
One kind of optical system of an information recording/reproducing apparatus for a magneto-optic disk is shown in FIG. 12.
Optical system A includes a laser beam emitting element 1, such as of a semiconductor for emitting a linearly polarized laser beam, a collimate lens 2 for collimating the beam, a beam forming prism 3 for correcting a sectional configuration of the beam, a half mirror prism 4, and an objective lens 5 for forming a spot on a magneto-optic disk D and taking out a reflecting beam reflected by the magneto-optical disk D. The optical system A includes a half-wave plate 6 for turning the azimuth of the linearly polarized light of the reflection beam guided through the objective lens 5 and the half mirror prism 4, and a polarizing beam splitter 9 for splitting the beam coming from the half-wave plate 6 and radiating to a light receiving element 7 for detecting tracking errors, and a light receiving element 8 for detecting focussing errors, respectively. Furthermore, the optical system A includes condenser lenses 10 and 10', which are disposed to the optical paths split by the polarizing beam splitter 9, and a cylindrical lens 11 disposed between the condenser lens 10 and the light receiving element 8 for detecting focussing errors.
The light receiving element 7 for detecting tracking errors performs a tracking error detection according to a so-called push-pull method, by finding a difference in the output between two adjacent light receiving areas. On the other hand, the light receiving element 8 for detecting focussing errors has four light receiving areas; these light receiving areas being arranged in the form of a matrix or grill pattern, i.e., the light receiving areas are arranged in two rows and two columns. The light receiving element 8 for detecting focussing errors performs a focussing error detection according to a so-called astigmatism method, by finding differences in the sums of the output of opposite angles of the four light receiving areas.
The reproduction of information recorded on the magneto-optic disk D utilizes a phenomenon in which when a light beam is reflected by the magneto-optic disk D, the polarizing direction (i.e., the direction of the vibrating surface of an electric field vector is very slightly turned, due to the magnetic Kerr effect.
That is, in the optical system A, a turn of the vibrating surface is converted into a difference in the intensity of light radiated to the respective light receiving elements 7 and 8 by utilizing the half-wave plate 6 and the polarizing beam splitter 9.
If the sum of the output of the two light receiving areas of the light receiving element 7 for detecting tracking errors is referred to as the "first output sum" and if the sum of the outputs of the four light receiving areas of the light receiving element 8 for detecting focussing errors is referred to as the "second output sum", a direct current portion, such as the output variation of the laser beam emitting element of a semiconductor, is offset by calculating the difference between the first output sum and the second output sum and only a signal portion caused by the turn of the beam in the polarizing direction is taken as a reproduction signal.
However, in such an optical system A of an information recording/reproducing apparatus for a magneto-optic disk, since the light receiving element 7 for detecting tracking errors and the light receiving element 8 for detecting focussing errors must be independently provided, the number of component parts becomes large and the optical system becomes bulky. Moreover, much time and labor are required for adjusting the system.